The present invention relates to microwave tunnel ovens and more particularly to an improved oven for the continuous processing of food products.
Microwave tunnel ovens provide an electrically conductive oven cavity whose size is on the order of several wave lengths of the microwave energy for the processing of packaged food products. French Pat. No. 1260933, dated April 4, 1961, describes a tunnel oven designed for continuous heating. It has a noria type handling system and is U-shaped to prevent the leakage of microwave radiation through its inlet and its outlet.
German Pat. No. 1116137 describes a straight line tunnel with a rectangular cross section, equipped with a system of wave traps, located at the inlet and the outlet. Their function is to insure that the microwave radiation remains inside the apparatus. It also features the shields located at the ends to absorb any microwave radiation that by-passes the traps themselves.
U.S. Pat. No. 3,263,052 describes equipment in which microwave energy is introduced through slots longitudinally spaced on a wave guide located on the upper section of the tunnel which has a rectangular cross section. The wave guide is parallel to the longitudinal axis of the tunnel. The radiation which is introduced through the slots, has a tendency to propagate towards the ends of the tunnel. A number of deflectors, made of metallic curtains, are attached transversally on the inner top surface of the oven and they extend vertically down to the top level of the product to be processed. Also, as in the above mentioned German patent, there are devices located at each end of the tunnel for the absorption of the microwave energy not absorbed in the tunnel itself. This patent also provides for injecting or extracting gases through slots in the wave guide or, as required, to plug up those openings with non-energy absorbing material to prevent condensation of the vapors caused by the process within the wave guide.
None of the apparatus described by the above patents can be used with non-packaged food stuff. These devices are not easily cleaned and unless they are kept clean and disinfected they will not meet hygenic standards.
The uni-directional injection system of microwave energy (from the top) used in the apparatus described by the above patents has the net effect of overheating the food in the area facing the energy sources, particularly when the foodstuff form a continuous layer the length of the oven. This condition of overheating is preferable from the viewpoint of optimum use of the apparatus as well as for the uniformity of heating, however, where the load is discontinuous, the extremities of the blocks have a tendency to heat up much faster than their central portion. When the pieces touch each other, as is the case for fishsticks or blocks of meat, the load can be considered as forming a continuous layer and when the loading is accomplished in that fashion, the edges and mostly the angles have a tendency to absorb more energy.
Other problems arise when the energy injection system includes a multitude of microwave sources such as magnetrons. These exhibit certain types of technical problems related to the energy coupling between the magnetrons that can cause their destruction. Also the ovens described in the above patents have a monolithic structure which makes their handling, as well as size modification difficult.